(1) Field of the Invention
This invention relates to a process for separating and recovering chlorine from a gaseous mixture comprising chlorine, carbon dioxide and non-condensable gas. This invention is also concerned with a process for removing chlorine from such a gaseous mixture.
Specifically, this invention relates to a process for separating and recovering chlorine from a gaseous mixture containing chlorine, carbon dioxide and non-condensable gas at high concentrations. This invention is also concerned with a process for recovering chlorine from a gaseous mixture containing chlorine, carbon dioxide and at least 50% by volume of non-condensable gas. Further, this invention also pertains to a process for absorbing and removing chlorine gas contained in carbon dioxide gas which is formed upon production or utilization of chlorine.
(2) Description of the Related Art
A variety of processes have already been proposed for the recovery of chlorine from a gaseous mixture containing chlorine, including the following patent publications:
(1) U.S. Pat. No. 3,972,691 discloses a process for recovering liquid chlorine from a gaseous mixture comprising 20-90% by volume of chlorine, 10-80% by volume of carbon dioxide, nitrogen, oxygen and carbon monoxide. The process comprises compressing the gaseous mixture at 4-8 atm, cooling and liquefying the compressed gaseous mixture in a rectification column of the total reflux type, and then adjusting the temperature of liquid chlorine collected in a bottom part of the rectification column to cause evaporation of carbon dioxide dissolved in the liquid chlorine.
(2) U.S. Pat. No. 2,199,797 discloses that an organic impurity contained at a concentration of 1% weight or lower in chlorine gas can be removed with liquid chlorine by bringing the chlorine gas into counter current contact with liquid chlorine in a washing column.
(3) U.K. Patent No. 938,073 discloses a process for separating a non-condensable impurity, which has a boiling point lower than chlorine and forms an explosive mixture with chlorine, from gaseous chlorine containing the impurity. The process comprises lowering stepwise the temperature of the gaseous chlorine and bringing the final gaseous residue into counter current contact with liquid chlorine, the temperature of which has been lowered to the condensing temperature of gaseous chlorine or lower, in a liquefaction column to liquefy gaseous chlorine, whereby chlorine is separated from the impurity of the low boiling point.
(4) U.S. Pat. No. 3,443,902 discloses a process to compress chlorine gas, which has been obtained by bringing an impurity-containing chlorine gas into counter current contact with liquid chlorine in a washing column and the impurity has hence been absorbed and removed by the liquid chlorine, thereby liquefying a portion of the chlorine gas by its heat exchange with the liquid chlorine in the washing column, and then to use the so-liquefied chlorine for the same purpose as the liquid chlorine mentioned above.
(5) U.K. Patent No. 1,164,069 discloses that a gaseous mixture composed of non-condensable gas, including nitrogen and chlorine, can be separated into liquid chlorine and the non-condensable gas by compressing the gaseous mixture to 6-10 atm, cooling the compressed gaseous mixture in two stages, and then cooling the cooled gaseous mixture further to -120.degree. F. to -150.degree. F. by heat exchange.
(6) U.S. Pat. No. 2,540,905 discloses a process for recovering chlorine in a form free of carbon dioxide. The process comprises absorbing chlorine from a liquefaction residual gas, which has been obtained after electrolysis of brine and contains 5-10% by weight of chlorine along with carbon dioxide, carbon monoxide, hydrogen, nitrogen, oxygen and other gaseous components, with a chlorinated solvent; and then causing carbon dioxide, which has been absorbed at the same time, to evaporate at a temperature higher than the absorption temperature by a method such as heating a lower part of an absorption column.
(7) U.S. Pat. No. 2,765,873 discloses a process for recovering chlorine in a form substantially free of non-condensable gas. The process comprises causing a solvent to absorb a gaseous mixture, which is composed of 30-50 wt.% of chlorine and air, under a pressure of 2.0-14.3 atm at a column top temperature of from -22.8.degree. C. to 32.2.degree. C. and a column bottom temperature higher by from 27.8.degree. C. to 52.8.degree. C. than the column top temperature.
(8) West German Patent No. 2413358 discloses a process to absorb chlorine alone from a mixture of chlorine and carbon dioxide gas as the hydrochlorite of an alkali metal and/or alkaline earth metal by using a multi-stage counter current absorption apparatus making use of an alkali metal hydroxide and/or an alkaline earth metal hydroxide and operating the apparatus while controlling the pH of the final stage of the liquid side at about 7.5.
The processes disclosed in the publications (1), (2), (3) and (4) respectively are applied where chlorine or chlorine and carbon dioxide as one or more condensable components are contained at relatively high concentrations. For the recovery of chlorine from a chlorine-containing gaseous mixture in which the concentration of non-condensable gas is about 50% by volume or even higher, these processes are accompanied by drawbacks due to the abundant existence of non-condensable gas.
These processes all employ counter current contact between descending liquid chlorine and ascending crude chlorine gas in a column although their purposes are for liquefaction, washing and distillation respectively and hence differ from each other. When non-condensable components are contained at high level in an ascending crude chlorine gas, it is thus impossible to avoid entrainment due to the ascending gas and a reduction in the efficiency of gas-liquid contact due to channeling of the descending liquid or a similar cause, thereby making it difficult to achieve their separation as intended.
If the amount of the descending liquid is increased with a view toward maintaining the efficiency, both the refrigeration load and heating load of the distillation column increase, and in addition, larger items of greater equipments--such as column main body, condenser and reboiler--must be used. This approach is therefore not economical. In the case of the process (4) the circulating load of non-condensable gas becomes enormous and the dew point of the compressed gas drops. If the compression pressure is low, there is the potential problem that chlorine cannot be liquefied upon its heat exchange with the washing column. Even if liquefaction is feasible, the power cost increases because of an increased compression ratio of the compressor and an increased circulating load so that the advantage of this process is lost.
Further, the processes (1) and (5) are basically intended for chlorine gas of a high concentration. A chlorine-containing gaseous mixture to be treated is compressed and cooled to liquefy chlorine for its separation. These processes are however intended for the recovery of high-purity chlorine. A waste gas separated from chlorine and composed principally of non-condensable gas therefore contains chlorine at a concentration as high as 5-9% by volume in the process (1) and 10% by volume even in the process (5).
The industrial recovery of chlorine poses an atmospheric pollution problem through the discharge of a waste gas containing chlorine at such high concentrations. To be ready for disposal as a waste, a gas must be free of chlorine. This however requires at least one chemical in a huge quantity for the removal of chlorine, to say nothing of facilities, and moreover, leads to a loss of chlorine. These processes are hence not economical. To reduce the chlorine content in a waste gas to such a low level that the chlorine may be ignored, it is indispensable to increase the compression pressure further and at the same time to lower the cooling and liquefying temperature further. This however leads to an increased power cost and also to an increased refrigeration cost. Moreover, it is not preferable for safety of facilities to compress a chlorine-containing gaseous mixture to a high pressure. In addition, it is not permissible to lower the cooling and liquefying temperature to the freezing temperature of carbon dioxide (-56.6.degree. C. at 5.2 atm) or lower so as to avoid blocking of facilities due to occurrence of dry ice. Such a liquefaction process cannot therefore avoid inclusion of chlorine at a certain concentration in a waste gas. The processes (6) and (7) both make use of a solvent, whereby an impurity is absorbed and then desorbed to recover chlorine. Of these, the process (6) includes heating a lower part of an absorption column to cause a portion of both the chlorine and a major portion of the carbon dioxide absorbed in the solvent to evaporate so that chlorine gas obtained in an evaporation column may have a higher purity. It is hence unavoidable that chlorine accompanies a waste gas from the top of the absorption column. In particular, where carbon dioxide is contained at a high level, it is necessary to intensify the heating of the lower part of the absorption column correspondingly. As a result, the chlorine concentration in the waste gas increases and the loss of chlorine and the solvent rises significantly.
The process (7) requires evaporation to be conducted under a substantially high pressure in an evaporation column because chlorine released from a solvent is recovered there by liquefaction. If the solvent absorbs more air than necessary in the absorption column, chlorine is recovered with a decreased purity. It is hence necessary to reduce the amount of the absorbent solvent. Due to this requirement, where carbon dioxide and non-condensable gas are contained at a high level, the absorption of chlorine cannot be achieved sufficiently and the levels of chlorine and the solvent accompanying the waste gas increase abruptly.
To lower the chlorine concentration in a waste gas by the solvent absorption method, the choices include increasing the amount of a solvent, lowering the temperature of the solvent or increasing the pressure of an absorption column further. It is however difficult to recover chlorine with a high purity because the absorption of carbon dioxide and non-condensable gas is promoted whichever method is relied upon.
Chlorine is a useful raw material produced industrially on a large scale primarily by brine electrolysis, and is used widely. Gaseous components are formed as byproducts upon production of chlorine. These components are accompanied by chlorine, so that due to the toxicity of chlorine, they cannot be released into the atmosphere without further treatment.
Accordingly, chlorine contained in such a gas is removed usually by its absorption with an alkaline substance. However, when another gas is contained besides chlorine and said another gas is acidic (like carbon dioxide gas) it is not only chlorine but also carbon dioxide that is absorbed in an alkaline solution. As a corollary to this, an alkali is required in an amount equal to the sum of the carbon dioxide and chlorine. In particular, where a gas contains carbon dioxide at a high level and chlorine at a trace level, an inconvenience arises in that a large amount of an alkali is required for the removal of such a trace amount of chlorine. It is accordingly desired to develop a process for the selective absorption and removal of chlorine which is contained in carbon dioxide.
The process (8) has already been proposed as a process for removing chlorine from a mixture of carbon dioxide gas and chlorine. In this process, pH 7.5 is however a value far higher than 6.35, which is the first dissociation constant (pKa) of carbonic acid, as shown in FIG. 4 and falls within the range where carbon dioxide gas reacts with an alkali metal hydroxide and/or alkaline earth metal hydroxide to form the corresponding bicarbonate.
To absorb chlorine alone from the mixture of carbon dioxide and chlorine in the above range, it is necessary to use an alkali exactly in an equimolar amount as chlorine. Where the concentration of chlorine varies, it is difficult to balance the alkali in quantity with chlorine.
In addition, pH 7.5 is close to the dissociation constant of hydrochlorous acid. When the pH becomes lower than the above level, hydrochlorous acid takes the form of a free acid so that it becomes prone to decomposition. Precise control of pH is therefore needed. Even if chlorine is removed successfully, the resulting hydrochlorite solution has strong oxidative effects and offensive odor and cannot thus be discarded as is. An additional step is therefore required for its reduction.